Occultation by (4) Vesta on Jan. 31 observed from S. America

Updated: 2008 Feb. 2 UT, 2h UT

     Observers in central Chile, northern Argentina, and Uruguay, 
had a chance to observe the occultation of SAO 159443 by (4) Vesta, 
the DAWN mission target, Wednesday morning, Jan. 31.  I put 
Steve Preston's finder charts in a Power Point file.  At least two 
observations, in Buenos Aires and Uruguay, were made of this event; 
their data are given below.

Vesta is so bright that few occultations of stars by it have been 
observed; the only really well-observed one was in January 1991 when 
the asteroid occulted a 7th-mag. star as seen from eastern North 
America.  The results from that occultation, and a plot of the 
observations in the plane of the sky at Vesta showing the shape of 
the asteroid can be seen in this figure.  Observations of an 
occultation of 8.8-mag. SAO 159443 = HIP 76876 in Libra by Vesta 
that occured at 7:13 U.T. January 31st (Wednesday morning) could 
help refine our knowledge of Vesta's 3-dimensional shape that in 
turn could be of help for the DAWN mission that is planning to send 
a spacecraft to orbit Vesta.  The wide (555 km) path of the 
occultation crossed major observatories in central Chile, northern 
Argentina, and Uruguay.  The occultation was expected to last 17 
seconds.  Since the star was fainter than Vesta, when the star 
disappeared, the combined brightness of Vesta plus the star dropped 
by only 0.3 magnitude with Vesta alone visible.  This small 
magnitude drop was hard to notice visually, but at least one visual 
observer in Buenos Aires did see and time the small brightening that 
occurred at reappearance.  A CCD drift scan observation was made in 
Uruguay; the data are here.  The star was at J2000 RA 15h 
41m 53.6s, Dec -12 deg. 45' 36".  More information about the 
occultation, including a regional map of the path and finder charts 
of several different scales to locate the star, are here with 
additional interactive maps and information on this Web page (scroll 
down to the line for Jan. 31 Vesta and click on the items to the 
right).  I have put Steve Preston's map of the path across South 
America, and his finder charts, in this Power Point file, which may 
be easier for some to view and print.  Methods for timing 
occultations with CCD drift scans are described by John Broughton 
here.

David Dunham, 2007 Feb. 2, 2h UT
home dunham@starpower.net 301-474-4722 cell 301-526-5590 
office & Blackberry david.dunham@jhuapl.edu 240-228-5609